1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly, to an array substrate for a wide viewing angle liquid crystal display (LCD) device and a method of manufacturing the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are driven based on optical anisotropy and polarization characteristics of a liquid crystal material. The LCD devices have been widely used for display units of portable electronic devices, monitors of personal computers, or televisions. Liquid crystal molecules have a long and thin shape, and the liquid crystal molecules are regularly arranged along in an alignment direction. Light passes through the LCD device along the long and thin shape of the liquid crystal molecules. The alignment of the liquid crystal molecules depends on the intensity or the direction of an electric field applied to the liquid crystal molecules. By controlling the intensity or the direction of the electric field, the alignment of the liquid crystal molecules is controlled to display images.
Generally, an LCD device includes two substrates, which are spaced apart and facing each other, and a liquid crystal layer is interposed between the two substrates. Each of the substrates includes an electrode. The electrodes from respective substrates face one another. An electric field is induced between the electrodes by applying a voltage to each electrode. An alignment direction of liquid crystal molecules changes in accordance with a variation in the intensity or the direction of the electric field.
However, since the LCD device uses a vertical electric field that is perpendicular to the substrates, the LCD device has poor viewing angles.
To resolve the poor viewing angles, an in-plane switching (IPS) mode liquid crystal display (LCD) device has been suggested.
In an IPS mode LCD device, a pixel electrode and a common electrode are alternately disposed on the same substrate, and a horizontal electric field that is parallel to the substrate is induced between the pixel electrode and the common electrode. Liquid crystal molecules are driven by the horizontal electric field and move parallel to the substrate. Accordingly, the IPS mode LCD device has the improved viewing angles.
However, the IPS mode LCD device has disadvantages of low aperture ratio and transmittance. To solve the disadvantages of the IPS mode LCD device, a fringe field switching (FFS) mode LCD device has been suggested.
FIG. 1 is a plan view of a pixel region of an array substrate for a related art FFS mode LCD device.
In FIG. 1, a gate line 43 is formed along a direction, and a data line 51 crosses the gate line 43 to define a pixel region P. A thin film transistor Tr is formed at the pixel region P and is connected to the gate line 43 and the data line 51. The thin film transistor Tr includes a gate electrode 45, a semiconductor layer 46 of an active layer 46a and ohmic contact layers 46b, a source electrode 55, and a drain electrode 58.
A pixel electrode 60 is formed at the pixel region P and is connected to the drain electrode 58 of the thin film transistor Tr. The pixel electrode 60 corresponds to substantially the whole pixel region P and has a rectangular plate shape.
A common electrode 75 is formed over the pixel electrode 60 with a passivation layer 60 interposed therebetweeen. The common electrode 75 includes openings oa in the pixel region P. The common electrode 75 extends into a next pixel region adjacent to the pixel region P and corresponds to substantially all over a display area including a plurality of pixel regions P. The openings oa have a bar shape and are parallel to the data line 51.
FIG. 2 is a cross-sectional view of a pixel region of an array substrate for a related art FFS mode LCD device. FIG. 2 corresponds to a cross-section taken along the line II-II of FIG. 1.
In FIG. 2, a gate electrode 45 and a gate line (not shown) are formed on a substrate 50. A gate insulating layer 30 covers the gate electrode 45 and the gate line. An active layer 46a is formed on the gate insulating layer 30 corresponding to the gate electrode 45, and ohmic contact layers 46b are formed on the active layer 46a and spaced apart from each other.
Source and drain electrodes 55 and 58 are formed on the ohmic contact layers 46b and spaced apart from each other.
A data line 51 is formed on the gate insulating layer 30 and crosses the gate line to define a pixel region P. A pixel electrode 60 is formed at the pixel region P and contacts an end of the drain electrode 58.
A passivation layer 70 is formed on the pixel electrode 60, and a common electrode 75 is formed on the passivation layer 70. The common electrode 75 is formed substantially all over a display area. The common electrode 75 includes openings oa corresponding to the pixel electrode 60. The common electrode 75 and the pixel electrode 60 overlap each other to form a storage capacitor.
In the array substrate for the related art FFS mode LCD device having the above-mentioned structure, when a voltage is applied to the pixel electrode 60 and the common electrode 75, a fringe field is induced between the pixel electrode 60 and the common electrode 75 overlapping each other. Therefore, even liquid crystal molecules over the electrodes are driven by the fringe field, and the FFS mode LCD device has a transmittance and an aperture ratio higher than the IPS mode LCD device.
By the way, in the FFS mode LCD device, the storage capacitor, which is formed by the pixel electrode 60 and the common electrode 75, is formed all over the pixel region P. Thus, the storage capacitor has three to five times as large capacitance as that of the IPS mode LCD device. If the storage capacitor has too large capacitance, it is hard to charge the storage capacitor in a high definition model or in a high frequency model.
To solve the problem, the width of the gate line 43 of FIG. 1 or the data line 51 should be increased to thereby decrease the resistance or the width of the channel of the thin film transistor Tr should be increased. However, this causes a decrease of the aperture ratio, and the transmittance and the brightness are lowered.